This invention relates generally to processes for production of xcex1-olefin polymers using unbridged fluxional metallocenes, primarily substituted aryl indenyl metallocenes, and more particularly to use of unbridged, fluxional, cyclopentadienyl or indenyl metallocene catalysts and catalyst systems in methods of production of high melting point olefin homo- and co-polymers, particularly elastomeric crystalline and amorphous block homo- and co-polymers of alpha olefins. More specifically, the invention is directed to: (1) the discovery and catalytic process use of a Polymerization Rate-Enhancement effect (PRE effect) in polymerization processes which involve the addition of minor amounts of ethylene to the polymerization system to produce polymers having properties ranging from crystalline thermoplastics to high melting point thermoplastic elastomers to amorphous gum elastomers, and methods for increasing polymerization production rates and polymer molecular weight; (2) the discovery and catalytic process use of an Elastomeric Property-Enhancement effect (EPE effect) in which small quantities of ethylene added to the polymerization system activates selected metallocene catalyst systems, which otherwise do not produce elastomeric polymers, to produce elastomeric polymers; and (3) novel substituted aryl indenyl metallocene catalysts.
Crystalline, amorphous, and elastic polypropylenes are known. Crystalline polypropylenes are generally regarded as comprising of predominantly isotactic or syndiotactic structures and amorphous polypropylene is regarded as comprising predominantly of an atactic structure. U.S. Pat. Nos. 3,112,300 and 3,112,301 both of Natta, et. al. describe isotactic and prevailingly isotactic polypropylene.
U.S. Pat. No. 3,175,199 to Natta et al. describes an elastomeric polypropylene which can be fractioned out of a polymer mixture containing prevailingly isotactic and atactic polypropylenes. When separated from the polymer mixture, a fraction of this polymer showed elastomeric properties which were attributed to a stereoblock structure comprising alternating blocks of isotactic and atactic stereosequences. U.S. Pat. No. 4,335,225 discloses a fractionable elastomeric polypropylene with a broad molecular weight distribution.
Elastomeric polypropylenes with narrow molecular weight distributions are also known which are produced in the presence of bridged metallocene catalysts. Polymers of this type were described by Chien et. al. in (J. Am. Chem. Soc. 1991, 113, 8569-8570), but their low melting point renders them unsuitable for certain applications. In addition, the activities of these catalyst systems are low.
U.S. Pat. No. 5,594,080 discloses an unbridged, fluxional metallocene catalyst system useful for the production of elastomeric polyolefins. These fluxional, unbridged catalysts can interconvert between geometric states on the time scale of the growth of a single polymer chain in order to produce isotactic, atactic stereoblock polyalphaolefins with useful elastomeric properties. Polyolefins produced with these fluxional catalysts systems can have a range of properties, from amorphous gum elastomers to useful thermoplastic elastomers to non-elastomeric thermoplastics.
The commercial utility of a catalyst system is closely tied to the polymerization activity. Processes that lead to an increase in activity of a polymerization system are of considerable practical utility. The activity of a polymerization system can in some cases be influenced by additives to the polymerization system. For example for both classical Ziegler-Natta systems as well as metallocene systems, the addition of hydrogen can result in an increase in propylene polymerization activity, see Pasquet, V., et al., Makromol. Chem. 1993, 194, 451-461 and references cited therein. One of the explanations for the hydrogen effect is the reactivation of the dormant sites resulting from 2,1-propylene misinsertions, see Corradini, P., et al., Makromol. Chem., Rapid Commun. 1992, 13, 15-20; Corradini, P., et al., Makromol. Chem., Rapid Commun. 1992, 13, 21-24; and Busico, V., et al., Makromol. Chem., Rapid Commun. 1993, 14, 97-103. Since hydrogen is also a chain transfer agent, the addition of hydrogen decreases the molecular weight, which limits the practical utility of the hydrogen effect where high molecular weight polymers are desired.
Activation of ethylene polymerization systems by the addition of small amounts of an alpha olefin is also known, see for example Brintzinger, H., et. al. Angew. Chemie, Int. Ed. Engl. 1995, 34, 1143-1170. This so-called xe2x80x9ccomonomer effectxe2x80x9d (see Spitz, R., et al. Makromol. Chem. 1988, 189, 1043-1050) is useful in a process for the synthesis of ethylene polymers, but not for alpha olefin polymers. Hefert, N., et. al. Makromol. Chem. 1993 194, 3167-3182 report no effect of hexene on the rate of propene polymerization with a metallocene catalyst. Several explanations have been forwarded to explain this xe2x80x9ccomonomer effectxe2x80x9d including a xe2x80x9ctrigger mechanismxe2x80x9d (Ystenes, M., Makromol. Chem. xe2x80x9cMacromolecular Symposiaxe2x80x9d 1993, 66, 71-81) and improved rates of diffusion due to the solubilization of active centers by incorporation of comonomer (see Koivumaki, J., et al. Macromolecules 1993, 26, 5535-5538).
Activation of propylene polymerization systems in the presence of 5% ethylene have been previously reported for magnesium chloride supported Ti-based catalysts by Spitz, R., et al. in Makromol. Chem. 1988, 189, 1043-1050 and in Spitz, R., et al. in xe2x80x9cTransition Metal Catalyzed Polymerizationxe2x80x9d, Quirk, R. P., Ed., Cambridge Univ. Press 1988, pp. 719-728, and with V-based Ziegler catalysts by Valvassori, A., et al. in Makromol. Chem. 1963, 61, 46-62. While such xe2x80x9csynergistic effectsxe2x80x9d have been observed with classical Ziegler-Natta catalyst systems, Koivurnaki et. al. point out that such synergistic effects do not work for homogeneous metallocene systems (see Koivumaki, J., et al. Macromolecules 1993, 26, 5535-5538).
Accordingly, there is a need for processes to improve the activity of metallocene catalysts systems capable of producing elastomeric polypropylenes of high molecular weight with high melting points.
We have discovered that the activity of fluxional unbridged metallocene polymerization catalysts containing at least one 2-arylindene ligand may be increased by the addition of small (typically 0.1-10 wt. %) amounts of ethylene to the polymerization system. In particular, the addition of ethylene to a propylene polymerization system derived from unbridged metallocene catalysts containing at least one 2-arylindene ligand results in a significant increase (up to ten-fold or above) in catalyst activity. We term this increase in activity the Polymerization Rate Enhancement effect (PRE), which can be measured in terms of an Ethylene Enhancement Factor (EEF) as a dimensionless ratio. Also, the molecular weight of the produced polymers may increase in the presence of ethylene. The amount of ethylene included in the reaction system can be selected and controlled to be so small as to result in essentially minimal ( less than 2 mole %) incorporation of ethylene units into the polymer, yet surprisingly results in a significant, disproportionately large increase in polymerization activity. More specifically, by addition of small amounts of ethylene into polypropylene reaction systems, an unexpectedly large (order of magnitude or more) increase in activity is achieved to produce elastomeric products.
Thus, in a first aspect of this invention, elastomeric olefin polymers are formed using unbridged fluxional, metallocene-based, catalyst systems in a polymerization process in which an activity-enhancing amount of ethylene is incorporated into the polymerization feed. This effect is herein termed the PRE effect, for Polymerization Rate-Enhancement effect, and is quantified as a dimensionless number in the range of from about 1.1 to about 10 or above, called the EEF for Ethylene Enhancement Factor. Typically, useful PRE (activity-enhancing) amounts of ethylene are above about 0.1 wt. % in the feed. Amounts of ethylene to generate the PRE effect may be greater than 0.5 wt. % and preferably range up to about 2 wt. %. However, if a polymer with more ethylene is desired, additional gethylene may be incorporated into the polymerization feed, including up to 10 to about 50 mole % based on olefin units.
Even though ethylene may be introduced into a polymer of this invention, only an activity-enhancing amount of ethylene for PRE is required, i.e., to increase the activity of the fluxional, metallocene-based, catalyst. Thus, a second important aspect of this invention is the ability to use a PRE activity-enhancing amount of ethylene in an olefin polymerization without substantially affecting the physical properties of the elastomer. Preferred elastomeric polymers containing ethylene linkages made according to this invention have high melting temperatures (as measured by DSC) above 80xc2x0 C., preferably about 100xc2x0 C., including in the range of from about 120xc2x0 C. to about 140xc2x0 C. or above.
In a third important aspect of this invention, we have discovered the ability to produce olefin (preferably propylene) elastomers through incorporation of ethylene using unbridged fluxional catalyst systems which may not otherwise produce acceptable elastomeric homopolymers. This effect is herein termed the EPE effect, for Elastomeric Property-Enhancing effect. The Elastomeric Property-Enhancing amount of ethylene required to produce such elastomers typically overlaps the aforesaid PRE activity-enhancing amount. Incorporation of up to about 5 mole % or more of ethylene typically will produce an elastomeric polymer using such catalyst systems. Typical useful amounts of incorporated ethylene include about 1 to 3 mole %. Again, a preferred polymer of this invention retains sufficient crystallinity to provide a high melting point (by DSC) of above 80xc2x0 C., preferably above 100xc2x0 C., including in the range of from about 120xc2x0 C. to about 140xc2x0 C. and above. For example, the novel Catalyst D of this invention bis [2-(3,5-trifluoromethylphenyl)indenyl] zirconium dichloride produces an elastomeric polypropylene with 9% ethylene incorporated in the polymer with a Tm of 100xc2x0 C. Even with ethylene contents of up to about 10 mole % or more, polymers of this invention typically show melting temperatures of 80xc2x0 C. and above in contrast to conventional propylene-ethylene copolymer elastomers produced by conventional catalysts which have a lower melting temperature.
Polymers of this invention show a broad melting range by DSC analysis and exhibit good elastic recoveries. The conventional measurement of the melting point (Tm) is the peak (or inverse peak) in the DSC curve. Polymers of this invention also typically retain properties after thermocycling of up to 100xc2x0 C. and above. By way of example, such polymers retain transparency after such a heat treatment and do not become opaque.
A preferred elastomeric I-olefin polymer of this invention is a propylene polymer in which an amount of ethylene is incorporated during polymerization such that the resulting elastomeric propylene polymer maintains sufficient physical properties at elevated temperature (such as melting temperature) to permit steam sterilization without deformation of a shaped article fabricated from the polymer. Typical steam sterilization conditions are maintenance of a temperature of 121xc2x0 C. or above at a 2 atmosphere steam pressure.
Further, in polymerization systems which produce thermoplastic crystalline propylene polymers, introduction of ethylene merely reduces the melting point. In contrast, the EPE effect of this invention unexpectedly results in converting the polymers to true elastomers while providing a method of control over melting point and retention of properties after thermocycling by adjustment of the ethylene content in the feed and end product polymers.
As noted above, the class of metallocenes of this invention is defined as xe2x80x9cfluxionalxe2x80x9d, meaning that the geometry of such metallocene can change between two isomeric states. This change in configuration occurs on a time scale that is slower than the rate of olefin insertion, but faster than the average time to construct (polymerize) a single polymer chain. The fluxional catalyst structure is such that upon isomerization the catalyst symmetry alternates between states that have different coordination geometries and thus different steroselectivities. The catalyst remains in that geometric symmetry for a time sufficient to be characterizable as a xe2x80x9cstatexe2x80x9d, before rotating or otherwise transforming to the other geometry or state. This geometric or state alternation can be controlled by selecting ligand type and structure to control rotation of the ligands on the ligand-metal bond. Further, through control of polymerization, precise control of the physical properties of the resulting polymers can be achieved.
This invention includes novel processes for tailoring block size distribution and resulting properties of the polymer such as: tacticity, molecular weight, molecular weight distribution, productivity, melt flow rate, melting point, crystallite aspect ratio, tensile set and tensile strength by varying the structure of the catalyst, the conditions of the polymerization reaction, and the solvents, reactants, additives and adjuvants employed, the latter adjuvants including use of ethylene in the PRE and EPE effect processes described above and in the examples.
The catalyst system of the present invention consists of the transition metal component metallocene in the presence of an appropriate cocatalyst. In broad aspect, the transition metal compounds have the formula: 
in which M is a Group 3, 4 or 5 Transition metal, a Lanthanide or an Actinide, X and Xxe2x80x2 are the same or different uninegative ligands, such as but not limited to hydride, halogen, hydrocarbyl, halohydrocarbyl, amine, amide, or borohydride substituents (preferably halogen, alkoxide, or C1 to C7 hydrocarbyl), and L and Lxe2x80x2 are the same or different substituted cyclopentadienyl or indenyl ligands, in combination with an appropriate cocatalyst. Exemplary preferred Transition Metals include Titanium, Hafnium, Vanadium, and, most preferably, Zirconium. An exemplary Group 3 metal is Yttrium, a Lanthanide is Samarium, and an Actinide is Thorium.
The ligands L and Lxe2x80x2 may be any mononuclear or polynuclear hydrocarbyl or silahydrocarbyl, typically a substituted cyclopentadienyl ring. Preferably L and Lxe2x80x2 have the formula: 
where R1, R2 and R3 may be the same or different substituted or unsubstituted alkyl, alkylsilyl or aryl substituents of 1 to about 30 carbon atoms, and R9 and R10 may be the same or different hydrogen, or substituted or unsubstituted alkyl, alkylsilyl, or aryl substituents of 1 to about 30 carbon atoms.
Ligands of this general structure include cyclopentadiene, and pentamethylcyclopentadiene. Other ligands L and Lxe2x80x2 of Formula 2 for the production of propylene-ethylene copolymers include substituted cyclopentadienes of the general formula: 
where R4-R10 have the same definition as R9 and R10 above. Preferred cyclopentadienes of Formula 3 include 3,4-dimethyl-1-phenyl-1,3-cyclopentadiene (R2=R3=CH3, and R6=H), 3,4-dimethyl-1-p-tolyl-1,3-cyclopentadiene (R2=R3=CH3, and R6=CH3), 3,4,-dimethyl-1-(3,5-bis(trifluoromethyl)phenyl)-1,3-cyclopentadiene (R2=R3=CH3, and R6=CF3), and 3,4-dimethyl-1-(4-tert-butylphenyl)-1,3cyclopentadiene (R2=R3=CH3, and R6=tBu).
Alternately preferred L and Lxe2x80x2 of Formula 1 include ligands wherein R1 is an aryl group, such as a substituted phenyl, biphenyl, or naphthyl group, and R2 and R3 are connected as part of a ring of three or more carbon atoms. Especially preferred for L or Lxe2x80x2 of Formula 1 for producing the homopolymers of this invention is a 2-arylindene of formula: 
Where R4, R5, R6, R7, R8, R9, R10, R11, R12, R13, and R14 may be the same or different hydrogen, halogen, aryl, hydrocarbyl, silahydrocarbyl, or halohydrocarbyl substituents. That is, R1 of Formula 2 is R4-R8-substituted benzene, and R2, R3 are cyclized in a 6-carbon ring to form the indene moiety. Particularly preferred 2-aryl indenes include: 2-phenylindene; 1-methyl-2-phenyl indene; 2-(3,5-dimethylphenyl)indene; 2-(3,5-bis-triflouromethylphenyl)indene; 2-(3,5-bis-tertbutylphenyl)indene; 2-(3,5-bis trimethylsilylphenyl)indene; 2-(4,-fluorophenyl)indene; 2-(2,3,4,5-tetrafluorophenyl)indene; 2-(2,3,4,5,6-pentaflourophenyl)indene; 2-(1-naphthyl)indene; 2-(2-naphthyl)indene; 2-[(4-phenyl)phenyl]indene; and 2-[(3-phenyl)phenyl]indene.
Preferred metallocenes according to the present invention include:
bis(2-phenylindenyl)zirconium dichloride;
bis(2-phenylindenyl)zirconium dimethyl;
bis(1-methyl-2-phenylindenyl)zirconium dichloride;
bis(1-methyl-2-phenylindenyl)zirconium dimethyl;
bis[2-(3,5-dimethylphenyl)indenyl]zirconium dichloride;
bis[2-(3,5-bis-trifluoromethylphenyl)indenyl]zirconium dichloride;
bis[2-(3,5-bis-tertbutylphenyl)indenyl]zirconium dichloride;
bis[2-(3,5-bis-trimethylsilylphenyl)indenyl]zirconium dichloride;
bis[2-(4,-fluorophenyl)indenyl]zirconium dichloride;
bis[2-(2,3,4,5,-tetraflorophenyl)indenyl]zirconium dichloride;
bis(2-(2,3,4,5,6-pentafluorophenyl)indenyl])zirconium dichloride;
bis[2-(1-naphthyl)indenyl]zirconium dichloride;
bis(2-(2-naphthyl)indenyl])zirconium dichloride;
bis(2-[(4-phenyl)phenyl]indenyl])zirconium dichloride;
bis[2-[(3-phenyl)phenyl]indenyl]zirconium dichloride;
(pentamethylcyclopentadienyl)(1-methyl-2-phenylindenyl)zirconium dichloride;
(pentamethylcyclopentadienyl)(2-phenylindenyl)zirconium dichloride;
(pentamethylcyclopentadienyl)(1-methyl-2-phenylindenyl)zirconium dirmethyl;
(pentamethylcyclopentadienyl)(2-phenylindenyl)zirconium dimethyl;
(cyclopentadienyl)(1-methyl-2-phenylindenyl)zirconium dichloride;
(cyclopentadienyl)(2-phenylindenyl)zirconium dichloride;
(cyclopentadienyl)(1-methyl-2-phenylindenyl)zirconium dimethyl;
(cyclopentadienyl)(2-phenylindenyl)zirconium dimethyl;
and the corresponding hafnium compounds such as:
bis(2-phenylindenyl)hafnium dichloride;
bis(2-phenylindenyl)hafnium dimethyl;
bis(1-methyl-2-phenylindenyl)hafnium dichloride;
bis(1-methyl-2-phenylindenyl)hafnium dimethyl;
bis[2-(3,5-dimethylphenyl)indenyl]hafnium dichloride;
bis[2-(3,5-bis-trifluoromethyphenyl)indenyl]hafnium dichloride;
bis[2-(3,5-bis-tertbutylphenyl)indenyl]hafnium dichloride;
bis[2-(3,5-bis-trimethylsilylphenyl)indenyl]hafnium dichloride;
bis[2,(4-fluorophenyl)indenyl]hafnium dichloride;
bis[2-(2,3,4,5-tetrafluorophenyl)indenyl]hafnium dichloride;
bis[2-(2,3,4,5,6-pentafluorophenyl)indenyl]hafnium dichloride;
bis[2-(1-naphthyl)indenyl]hafnium dichloride;
bis[2-(2-naphthyl)indenyl]hafnium dichloride;
bis(2-((4-phenyl)phenyl)indenyl])hafnium dichloride;
bis[2-[(3-phenyl)phenyl]indenyl]hafnium dichloride;
(pentamethylcyclopentadienyl)(1-methyl-2-phenylindenyl)hafnium dichloride;
(pentamethylcyclopentadienyl)(2-phenylindenyl)hafnium dichloride;
(pentamethylcyclopentadienyl)(1-methyl-2-phenylindenyl)hafnium dimethyl;
(pentamethylcyclopentadienyl)(2-phenylindenyl)hafnium dimethyl;
(cyclopentadienyl)(1-methyl-2-phenylindenyl)hafnium dichloride;
(cyclopentadienyl)(2-phenylindenyl)hafnium dichloride;
(cyclopentadienyl)(1-methyl-2-phenylindenyl)hafnium dimethyl;
(cyclopentadienyl)(2-phenylindenyl)hafnium dimethyl;
and the like.
Other metallocene catalyst components of the catalyst system according to the present invention include:
bis(3,4-dimethyl-1-phenyl-cyclopentadienyl)zirconium dichloride;
bis(3,4-dimethyl-1-p-tolyl-cyclopentadienyl)zirconium dichloride;
bis(3,4-dimethyl-1-(3,5 bis(trifluoromethyl)phenyl)-cyclopentadienyl)zirconium dichloride;
bis(3,4-dimethyl-1-(4-tert-butylphenyl)-cyclopentadienyl)zirconium dichloride;
and the corresponding hafnium compounds, such as:
bis(3,4-dimethyl-1-phenyl-cyclopentadienyl)hafnium dichloride;
bis(3,4-dimethyl-1-p-tolyl-cyclopentadienyl)hafnium dichloride;
bis(3,4-dimethyl-1-(3,5 bis(trifluoromethyl)phenyl)-cyclopentadienyl)hafnium dichloride;
bis(3,4-dimethyl-1-(4-tert-butylphenyl)-cyclopentadienyl)hafnium dichloride;
and the like.
It should be understood that other unbridged, rotating, non-rigid, fluxional metallocenes may be employed in the methods of this intention, including those disclosed in our above-identified Provisional applications, which are hereby incorporated by reference to extent needed for support.
The Examples disclose a method for preparing the metallocenes in high yield. Generally, the preparation of the metallocenes consists of forming the indenyl ligand followed by metallation with the metal tetrahalide to form the complex.
Appropriate cocatalysts include alkylaluminum compounds, methylaluminoxane, or modified methylaluminoxanes of the type described in the following references: U.S. Pat. No. 4,542,199 to Kaminsky, et al.; Ewen, J. Am. Chem. Soc., 106 (1984), p. 6355; Ewen, et al., J. Am. Chem. Soc. 109 (1987) p. 6544; Ewen, et al., J. Am. Chem. Soc. 110 (1988), p. 6255; Kaminsky, et al, Angew. Chem., Int. Ed. Eng. 24 (1985), p. 507. Other cocatalysts which may be used include Lewis or protic acids, such as B(C6F5)3 or [PhNMe2H]+B(C6F5)xe2x88x924, which generate cationic metallocenes with compatible non-coordinating anions in the presence or absence of alkyl-aluminum compounds. Catalyst systems employing a cationic Group 4 metallocene and compatible non-coordinating anions are described in European Patent Applications 277,003 and 277,004 filed on Jan. 27, 1988 by Turner, et al.; European Patent Application 427,697-A2 filed on Oct. 09, 1990 by Ewen, et al.; Marks, et al., J. Am. Chem. Soc., 113 (1991), p. 3623; Chien, et al., J. Am. Chem. Soc., 113 (1991), p. 8570; Bochmann et al., Angew. Chem. Intl. Ed. Engl. 7 (1990), p. 780; and Teuben et al., Organometallics, 11 (1992), p. 362, and references therein.
The catalysts of the present invention consist of un-bridged, non-rigid, fluxional metallocenes which can change their geometry on a time scale that is between that of a single monomer insertion and the average time of growth of a polymer chain. This is provided by a non-rigid metallocene catalyst comprising cyclopentadienyl and/or substituted cyclopentadienyl ligands substituted in such a way that they can alternate in structure between states which have different coordination geometries. This is achieved in the present invention by using unbridged cyclopentadienyl ligands.
In one of many embodiments, these catalyst systems can be placed on a suitable support such as silica, alumina, or other metal oxides, MgCl2 or other supports. These catalysts can be used in the solution phase, in slurry phase, in the gas phase, or in bulk monomer. Both batch and continuous polymerizations can be carried out. Appropriate solvents for solution polymerization include liquified monomer, and aliphatic or aromatic solvents such as toluene, benzene, hexane, heptane, diethyl ether, as well as halogenated aliphatic or aromatic solvents such as CH2Cl2, chlorobenzene, fluorobenzene, hexaflourobenzene or other suitable solvents. Various agents can be added to control the molecular weight, including hydrogen, silanes and metal alkyls such as diethylzinc.
The metallocenes of the present invention, in the presence of appropriate cocatalysts, are useful for the homo-polymerization (and co-polymerization) of alpha-olefins, such as propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-octene, and combinations thereof, and of copolymerization with ethylene. The polymerization of olefins is carried out by contacting the olefin(s) with the catalyst systems comprising the transition metal component and in the presence of an appropriate cocatalyst, such as an aluminoxane, or a Lewis acid such as B(C6F5)3. In co-monomer systems, and in particular ethylene-propylene monomer systems, productivities in excess of 41 kg/g for the copolymerizations has been attained [see Example 85]
The metallocene catalyst systems of the present invention are particularly useful for the polymerization of propylene monomers and propylene-ethylene monomer mixtures to produce polypropylenes and propylene-ethylene co-polymers with novel elastomeric properties. By elastomeric, we mean a material which tends to regain its shape upon extension, such as one which exhibits a positive power of recovery at 100%, 200% and 300% elongation. The properties of elastomers are characterized by several variables. The tensile set (TS) is the elongation remaining in a polymer sample after it is stretched to an arbitary elongation (e.g. 100% or 300%) and allowed to recover. Lower set indicates higher elongational recovery. Stress relaxation is measured as the decrease in stress (or force) during a time period (e.g. 30 sec. or 5 min.) that the specimen is held at extension. There are various methods for reporting hysteresis during repeated extensions. In the present application, retained force is measured as the ratio of stress at 50% elongation during the second cycle recovery to the initial stress at 100% elongation during the same cycle. Higher values of retained force and lower values of stress relaxation indicate stronger recovery force. Better general elastomeric recovery properties are indicated by low set, high retained force and low stress relaxation.
It is believed that the elastomeric properties of the polypropylenes and propylene-ethylene copolymers of this invention are due to an alternating block structure comprising of isotactic and atactic stereo-sequences. Without being bound by theory, it is believed that isotactic block stereosequences tightly interlocked with one another provide crystalline blocks which can act as physical crosslinks in the polymer network. These crystalline blocks are separated from one another by intermediate, atactic lengths of the polymer which enable the polymer to elastically deform. While we do not wish to be bound by theory it is believed the ethylene is incorporated randomly between the propylene units in the chain.
The structure of the polymers may be described in terms of the isotactic pentad content [mmmm] which is the percentage of isotactic stereosequences of 5 contiguous stereocenters, as determined by 13C NMR spectroscopy (Zambelli, A., et al. xe2x80x9cModel Compounds and 13C NMR Observation of Stereosequences of Polypropylenexe2x80x9d Macromolecules 1975, 8, 687-689). The isotactic pentad content of statistically atactic polypropylene is approximately 6.25%, while that of highly isotactic poly-propylene can approach 100%. For co-polymers the isotactic pentad content may be defined as the ratio of the area of PmPmPmPmPm+PmPmPmPmPE peaks over the area of all methyl peaks.
While it is possible to produce propylene homopolymers and copolymers with ethylene having a range of isotactic pentad contents, the elastomeric properties of the polymer will depend on the distribution of isotactic (crystalline) and atactic (amorphous) stereosequences, as well as the distribution of comonomer in the copolymer. Semicrystalline thermoplastic elastomers of the present class of materials consist of amorphous-crystalline block polymers, and thus the blockiness of the polymer determines whether it will be elastomeric. Crystallizable isotactic block length and content must be sufficient to provide a crosslinked network with usefully high Tm, but below the crystallinity of a hard plastic.
We have discovered that the structure, and therefore the properties of the alpha olefin polymers obtained with the catalysts of the present invention are dependent on olefin concentration, the ratio of olefins in the feed, the nature of the ligands, reactant pressure, the temperature of the polymerization, the nature of the transition metal, the ligands on the metallocene, the nature of the cocatalyst, and the reaction system.
It will be appreciated from the illustrative examples that the catalyst systems of this invention provide a broad range of polymer properties from the polymerization process of this invention. Polymers which range in properties from non-elastomeric thermoplastics to thermoplastic elastomers can be readily obtained by suitable manipulation of the metallocene catalyst, the reaction conditions, or the cocatalyst to give all by proper choice of process conditions and catalyst.
Without being bound by theory, it is believed that it is critical for the present invention to have a catalyst which can isomerize between states on a time scale that is slower than the rate of olefin insertion but faster than the average time to construct a single polymer chain in order to obtain a block structure. In addition, to produce elastomeric polymers, the catalyst complex isomerizes between states which have different coordination geometries. This is provided in the present invention by metallocene catalysts comprising of unbridged cyclopentadienyl-based ligands which are substituted in such a way that they can exist in different geometric states during the course of the polymerization reaction.
Based on the evidence to date, it appears that the rotation of the cyclopentadienyl ligands provides a mechanism for the alternation of catalyst geometry between the two states. The average block size distribution for a polymer produced with a catalyst which can change its state is controlled by the relative rate of polymerization versus catalyst isomerization as well as the steady-state equilibrium constant for the various coordination geometries (e.g. chiral vs. achiral). The catalysts of this invention provide a means of producing polypropylenes and other alpha olefins with a wide range of isotactic and atactic block lengths by changing the substituents on the cyclopentadienyl ligands of the metallocene. It is believed that modification of the cyclopentadienyl ligands and/or the nature of the transition metal will alter one or more of the following: The rate of polymerization, the rate of catalyst isomerization, and the steady-state equilibrium constant between the various coordination geometries, all of which will affect the block lengths and block length distribution in the resulting polymer. For example, it is believed that introduction of larger substituents on the cyclopentadienyl ligands will slow the rate of rotation and thereby increase the block lengths in the polymer. Of particular interest is the ability to produce high melting thermoplastic elastomers from catalysts which normally produce only non-elastomeric thermoplastics by the incorporation of small amounts of a second olefin. The comonomer is believed to insert randomly into the isotactic and atactic blocks, thereby disrupting crystallinity, but still providing a thermoplastic elastomer network (i.e. alternating stereoblock structure) with sufficient isotactic block lengths to achieve high melting polymers.
As described in U.S. Pat. No. 5,594,080, the disclosure of which is incorporated by reference herein, fluxional catalysts of the type described herein are useful for the production of elastomeric polyolefins. The productivity of catalyst systems has a large influence on their commercial viability; we have found that addition of small amounts of ethylene to a reaction system useful for the preparation of polyolefin elastomers has a quite unexpected and beneficial effect of increasing the productivity of the reaction system dramatically. It is known that most polymerization catalysts are more active for ethylene polymerization than alpha olefin polymerization, thus a somewhat higher productivity might be expected for a polymerization system containing both ethylene and an alpha olefin. However, the disproportionately large and unexpected increase in productivity of the catalysts systems of the present invention in the presence of as little as 0.6 weight percent ethylene in the reaction system is not predictable from prior works, and is evidence for the non-linear increase in productivity which we term the xe2x80x9cPolymerization Rate-Enhancement effectxe2x80x9d (PRE effect), which is quantifiable in terms of an xe2x80x9cEthylene Enhancement Factorxe2x80x9d or EEF. The ethylene enhancement factor EEF can be calculated from the well-known equations which describe the copolymerization of olefins. By way of illustration, we derive these equations for the effect of ethylene on propylene polymerization, but the Polymerization Rate-Enhancement effect will apply in the case of other alpha olefins as well.
The rate of olefin polymerization in the presence of hvo monomers such as ethylene and propylene can be described by 1st order Markov model given by the following four equations: 
where Me and Mp are the active centers with the last ethylene and propylene inserted units, respectively and kee is the rate constant for ethylene insertion at an ethylene site Me, and kep is the rate constant of propylene insertion at an ethylene site Me, etc . . . . The rate of polymerization in the presence of both ethylene and propylene, Rep, can be expressed as:
Rep=kee[Me][E]+kpe[Mp][E]+kep[Me][P]+kpp[Mp][P]
Since, under steady-state conditions, the rates of interconversion of Me into Mp and of Mp into Me are equal:
kpe[Mp][E]=kep[Me][P]
and we can express [Me] through [Mp] as:
Rep=kee(kpe/kep)[Mp][E][E]/[P]+2kpe[Mp][E]+kpp[Mp][P].
At low ethylene concentrations in the feed, where [P]=[Ph], the ratio of the rate of polymerization in the presence of both monomers, Rep, to the rate of propylene polymerization, Rpp, is:       Rep    Rpp    =      xe2x80x83    ⁢                                          re            ⁡                          [              E              ]                                2                                      re            ⁡                          [              P              ]                                2                    ⁢                        kpp          ⁡                      [            Mp            ]                                                kpp            h                    ⁡                      [                          M              p              h                        ]                                +                            2          ⁡                      [            E            ]                          ⁢                  kpp          ⁡                      [            Mp            ]                                                rp          ⁡                      [            P            ]                          ⁢                              kpp            h                    ⁡                      [                          M              p              h                        ]                                +                  kpp        ⁡                  [                      M            p                    ]                                      kpp          h                ⁡                  [                      M            p            h                    ]                    
where kpph, Mph, Ph signify the values for propylene polymerization in the absence of ethylene, and kee/kep=re and kpp/kpe=rp.
The ratio of the rates of polymerization in the presence and absence of ethylene are related to the ratio of the corresponding productivities, Pep and Ppp:                               Rep          Rpp                =                              Pep            Ppp                    =                                                    kpp                ⁡                                  [                  Mp                  ]                                                                              kpp                  h                                ⁡                                  [                                      M                    p                    h                                    ]                                                      ⁢                                          (                                                                                                    re                        ⁡                                                  [                          E                          ]                                                                    2                                        /                                                                  rp                        ⁡                                                  [                          P                          ]                                                                    2                                                        +                                                            2                      ⁡                                              [                        E                        ]                                                              /                                          (                                              rp                        ⁡                                                  [                          P                          ]                                                                    )                                                        +                  1                                )                            .                                                          Eq        .                  xe2x80x83                ⁢                  (          5          )                    
In equation 5 the increase of polymerization productivity due to the faster rate of ethylene insertion is described by expression (re[E]2/rp[P]2+2[E]/(rp[P])+1). The expression kpp[Mp]/kpph[Mph] is the Ethylene Enhancement Factor (EEF) and describes the ratio of the rate of consecutive propylene-propylene insertions in the presence of ethylene as compared to the rate in the absence of ethylene:                     EEF        =                                            kpp              ⁡                              [                Mp                ]                                                                    kpp                h                            ⁡                              [                                  Mp                  h                                ]                                              =                                    Pep              Ppp                        ⁢                                                            (                                                                                                              re                          ⁡                                                      [                            E                            ]                                                                          2                                            /                                                                        rp                          ⁡                                                      [                            P                            ]                                                                          2                                                              +                                                                  2                        ⁡                                                  [                          E                          ]                                                                    /                                              (                                                  rp                          ⁡                                                      [                            P                            ]                                                                          )                                                              +                    1                                    )                                                  -                  1                                            .                                                          Eq        .                  xe2x80x83                ⁢                  (          6          )                    
If there is no ethylene enhancement effect, then the EEF should be equal to unity, that is EEF=kpp[Mp]/(kpph[Mph])=1. An EEF greater than one is a metric that signifies an unexpected and non-linear increase in productivity in the presence of ethylene that cannot be anticipated due to the greater rate of ethylene insertion relative to that of alpha olefins.
For the catalysts of the present invention, we find dramatic and non-linear increases in productivities of alpha olefins in the presence of minor amounts of ethylene. Catalyst systems containing as little as 0.6% by weight of ethylene in the feed result in more than a two-fold increase in productivity (EEF=2.2) to give elastomeric polyolefins. In another example, as little as 5 wt. % ethylene in the feed results a 10-fold increase in productivity to give useful elastomeric products. Thus, one of the benefits of the Polymerization Rate-Enhancement effect (PRE effect) of this invention is that catalyst systems which in the absence of ethylene might be of marginal or little commercial interest are useful for the production of elastomeric polymers.
By using the novel metallocene catalyst systems of the invention without ethylene, we obtain polymers which range in properties from non-elastomeric thermoplastics to useful elastomeric xcex1-olefin polymers. By use of ethylene as described herein to take advantage of the Polymerization Rate-Enhancement effect, elastomeric xcex1-olefin polymers may be obtained, but at rates up to ten fold greater than the use of the same catalyst systems without the ethylene.
Furthermore, by use of the novel metallocene catalysts of the present invention we obtain alpha olefin (preferably propylene) elastomers by adding ethylene to an unbridged fluxional metallocene polymerization system which may not otherwise produce useful elastomeric homopolymers. By use of certain metallocene catalysts of the present invention, a remarkable improvement in the properties of propylene homopolymers can be realized by incorporation of small amounts of ethylene (10% or less). For example, polymerization of propylene with certain catalysts of the present invention can yield non-elastomeric propylene homopolymers with tensile sets above 39%, stress relaxation above 60% and no retained force. Incorporation of small amounts (10% or less) of ethylene into these polymerization system surprisingly results in polymers with good elastomeric properties. This is an example of the Elastomeric Property-Enhancement (EPE) effect.
The polymers of the present invention have useful elastomeric properties. These are a consequence the degree of crystallinity in the polymers which is controlled by the use of the catalysts and ethylene enhancement processes of this invention. The degree of crystallinity of the polymers of this invention are typically in the range of 1-40%, preferably in the range of 5-30% and most preferably in the range of 10-25%. The melting points of the polymers of the present invention are typically above 80xc2x0 C., preferably above 100xc2x0 C., including those in the range of 120xc2x0 C. to 140xc2x0 C. and above. The elastomeric polymers of the present invention exhibit tensile moduli in the range of 2-30 MPa, with values preferably below 20 MPa and most preferably below 15 MPa. The elastomeric polymers of the invention exhibit a positive force of recovery upon elongation. Typically, the retained force at 50% extension following 100% elongation is in the range of 10-50%, preferably in the range of 15-50%, and most preferably in the range 20-50%. The recovery properties of the polymers are also very good as evidenced by tensile set at 100% elongations of typically less than 50%, preferably less than 20% and most preferably less than 10%.